Rich land, adequate rainfall (32-36 in./81-91 cm annually), and a long growing season make Illinois an important agricultural state. It consistently ranks among the top states in the production of corn and soybeans. Hogs and cattle are also principal sources of farm income. Other major crops include hay, wheat, and sorghum. Beneath the fertile topsoil lies mineral wealth, including fluorspar, bituminous coal, and oil; Illinois ranks high among the states in the production of coal, and its reserves are greater than any other state east of the Rocky Mts.

Its agricultural and mineral resources, along with its excellent lines of communication and transportation, made Illinois industrial; by 1880 income from industry was almost double that from agriculture. Leading Illinois manufactures include electrical and nonelectrical machinery, food products, fabricated and primary metal products, and chemicals; printed and published materials are also important.

Metropolitan Chicago, the country's leading rail center, is also a major industrial, as well as a commercial and financial, center. Suburbs of Chicago such as Schaumburg and Oak Brook have become important business centers. Scattered across the northern half of the state are cities with specialized industries Elgin , Peoria, Rock Island , Moline , and Rockford. Industrially important cities in central Illinois include Springfield and Decatur.